In the past there has been rather extensive description of electrophoretic migration imaging processes. In these processes, as set forth in Sugarman, U.S. Pat. No. 2,758,939 issued Aug. 14, 1956, a layer of photoconductive particles, that is, electrically photosensitive particles, is subjected to the influence of an electric field between two spaced electrodes, at least one of which is transparent, and is exposed to a light image. As a result, those particles which are in electrical contact with an electrode during light exposure undergo a net change in charge polarity and migrate or are attracted to the adjacent spaced electrode having the opposite polarity, whereas the particles which are not exposed to light retain their original charge polarity. Thus, by producing an imagewise change in the charge polarity of the exposed photoconductive particles, photoconductive particle images may be formed at or near the surfaces of one or both of the two spaced electrodes which correspond to the original light image. According to one embodiment of the Sugarman process, the electrode surface at which the exposed electrically photosensitive particles undergo a net change in charge polarity is transparent and electrically conductive and the other spaced electrode surface bears a thin electrically insulating layer (sometimes referred to as a "blocking" layer) to prevent or at least reduce charge exchange with the electrically photosensitive particles.
Since the publication of the above-described electrophoretic migration imaging process appearing in U.S. Pat. No. 2,758,939, a number of other publications have appeared relating to the electrophoretic migration imaging process described by Sugarman. These publications include, for example, Uhrig, British Pat. No. 950,297 published Feb. 26, 1964, a series of U.S. patents by E. K. Kaprelian including No. 2,940,847 issued June 14, 1960; No. 3,100,426 issued Aug. 13, 1963; No. 3,140,175 issued July 7, 1964; and No. 3,143,508 issued Aug. 4, 1964. More recently, a number of other publications have appeared relating to the electrophoretic migration imaging process described by Sugarman, Uhrig, and Kaprelian such as Tulagin et. al. U.S. Pat. No. 3,384,565 issued May 21, 1968; Tulagin et. al. U.S. Pat. No. 3,615,558 issued Oct. 26, 1971; Clark, U.S. Pat. No. 3,384,566 issued May 21, 1968; Yeh U.S. Pat. No. 3,383,993 issued May 21, 1968; Jelfo, U.S. Pat. No. 3,616,398 issued Oct. 26, 1971; Wells, U.S. Pat. No. 3,647,660, issued Mar. 7, 1972; and Walsh, Canadian Pat. No. 899,137 issued May 2, 1972. These known electrophoretic migration imaging processes of the type described by Sugarman, Uhrig, and Kaprelian are often referred to herein as PhotoElectroPhoretic processes or the acronym PEP.
To applicant's knowledge, in known PEP processes the exposed photosensitive particles which are in electrical contact with an electrode, sometimes referred to as an "injecting" electrode, undergo a net change in their charge polarity to produce photosensitive particles of one polarity in exposed image areas while the unexposed photosensitive particles having an opposite polarity remain in non-image or dark areas. As a result, the photosensitive particles in the exposed areas which undergo a net change in charge polarity migrate or are attracted under the influence of the electric field to the spaced electrode having the opposite polarity, and the unexposed photosensitive particles which retain their original charge polarity migrate or are attracted to the other spaced electrode.
As suggested above, a primary difficulty encountered in PEP processes known to applicant is that in these processes it is the exposed photosensitive particles which undergo a net change in charge polarity. In many situations, however, it would be desirable to cause the unexposed particles to undergo a net change in charge polarity. If this could be done, it would be possible to obtain images having a reverse image snese in comparison to the images typically formed in known PEP processes.
As described in Jelfo, U.S. Pat. No. 3,616,398, issued Oct. 26, 1971 (at col. 1, lines 34-42), Wells, U.S. Pat. No. 3,647,660, issued Mar. 7, 1972 (at col. 1, lines 48-54), and Walsh, Canadian Pat. No. 899,137 issued May 2, 1972 (at page 2, line 24 - pg. 3, line 14), in conventional PEP monochrome and multicolor processes, acceptable imaging has been found to be generally restricted to a single sense process so that with positive input a negative image is produced and vice versa.
The processes described in the Jelfo and Wells patents referenced above attempt to overcome the above-described image sense restrictions encountered in conventional PEP monochrome processes. However, the processes described by Jelfo and Wells for attempting to provide image sense reversal in known PEP processes are quite restricted in their applicability because these processes require, at the very least, the addition of either beta-carotene, a vitamin precursor, or a halogen to the admixture of photosensitive particles used in these processes.
The process described in the Walsh Canadian patent referenced above attempts to overcome the above-described image sense restrictions encountered in conventional multicolor PEP processes by improving the quality of the color image formed on the electrode surface opposite the injecting electrode used in conventional PEP multicolor processes. According to the Walsh Canadian patent this may be done by attempting to eliminate or at least reduce the number of "undesired" photosensitive particles which are present in conventional multicolor PEP processes due to the bipolar properties of these particles.
As will be apparent from the disclosure of the Jelfo, Wells, and Walsh patents, the particle migration mechanism which occurs in the processes described by Jelfo, Wells, and Walsh does not differ from known PEP processes. That is, in the PEP processes described by Jelfo, Wells, and Walsh as in conventional PEP processes, it is the exposed photosensitive particles which undergo a net change in charge polarity whereas the unexposed particles retain their original charge polarity.
Thus, the basic exposure and charge exchange mechanism undergone by the exposed and unexposed electrically photosensitive particles, respectively, employed in the Jelfo, Walsh, and Wells processes is essentially the same as that of conventional PEP processes. Accordingly, the attempts of Jelfo, Walsh, and Wells to achieve image sense reversal of conventional PEP processes in no way provide an electrophoretic migration imaging process wherein the unexposed particles are caused to undergo a net change in charge polarity.